如何使用Jest模拟直接导入的函数？

Question

I'm trying to verify that my method is correctly invoking another, imported method. For the life of me, I can't figure out how to mock the imported method using Jest.

Method I want to test

LandingPageManager.ts

import {getJSON} from './getJSON';

public fetchData(url: string) {
    getJSON(url);
}

Method I want to mock

getJSON.ts

export function getJSON(url: string) {
    // XHR requests logic 
}

Test method

LandingPageManager.test.ts

import 'jest';
import {getJSON} from '../../../src/web/getJSON';
import {LandingPageManager} from '../../../src/web/LandingPageManager';

describe('fetchData', () => {
  let manager = new LandingPageManager();
  it('passes the correct URL to getJSON', () => {
    const getJsonSpy = jest.mock('../../../src/web/getJSON', jest.fn());

    manager.fetchData('sampleValue');
    expect(getJsonSpy).toHaveBeenCalledWith('sampleValue');

    getJsonSpy.restoreAllMocks();
  });
});

Error I'm getting

 jest.fn() value must be a mock function or spy

I've tried setting up the mock a variety of different ways. But I can't seem to get the syntax right.

Can anyone help point me in the right direction? I feel like this should be possible.

Answer 1

Finally figured out the answer.

Nothing needed to change in the source code (either the imported module or the class under test).

The import needed to change from:

import {getJSON} from '../../../src/web/getJSON';

to:

import * as getJSON from '../../../src/web/getJSON';

And then I was able to directly specify the function for spying with:

const jsonSpy = jest.spyOn(getJSON, 'getJSON');

Fixed test case

Here's how it all works together now.

LandingPageManager.test.ts

import 'jest';
// **** 1.) Changed the below line: ****
import * as getJSON from '../../../src/web/getJSON';
import {LandingPageManager} from '../../../src/web/LandingPageManager';

describe('fetchData', () => {
  let manager = new LandingPageManager();
  it('passes the correct URL to getJSON', () => {
    // **** 2.) Can now specify the method for direct mocking ****
    const jsonSpy = jest.spyOn(getJSON, 'getJSON');

    manager.fetchData('sampleValue');
    expect(getJsonSpy).toHaveBeenCalledWith('sampleValue');

    getJsonSpy.restoreAllMocks();
  });
});